In production of semiconductor wafers, a polishing process is applied to the wafers using a polishing apparatus for the purpose of finishing a wafer surface to a mirror-smooth surface that is free of concave/convex and has a high flatness.
FIG. 1 are schematic diagrams showing a polishing process of a semiconductor wafer by means of a conventional polishing apparatus, in which FIG. 1(a) is a front view and FIG. 1(b) is a top view. The polishing apparatus shown in the said figure is composed of a work carrier 4 for retaining a wafer and a table 2 equipped with a polishing cloth 3.
In a polishing process of the wafer 5 using the polishing apparatus shown in FIG. 1, after the wafer 5 is retained by the work carrier 4, the work carrier 4 and the table 2 are rotated in the direction indicated by outlined arrows in FIG. 1(b) to perform polishing while pressing the wafer 5 against the polishing cloth 3 by applying load. During the polishing process, frictional forces resulting from rotation of the work carrier 4 as well as the table 2 incurred by friction between the pressed wafer 5 and the polishing cloth 3 are respectively exerted on the work carrier 4 and the table 2.
Because the frictional forces exerted on the work carrier and the table become a factor for contributing to vibrations of the work carrier, the table, or the entire apparatus, an increase in frictional force causes the vibrations to be more likely generated, and results in the generation of further intensified vibrations.
In the polishing process, the operation of starting polishing is performed, in some cases, by causing the table and work carrier both being at rest to rotate in a condition that polishing cloth and the wafer have been pressed against each other, or in other cases, after the table and the work carrier are rotated while the polishing cloth and the wafer are still apart from each other, it is performed by pressing the polishing cloth and the wafer against each other. In the latter cases, a load is immediately generated at the instant when the polishing cloth and the work carrier are pressed against each other, and the load is increased much more as a wafer diameter or a table diameter becomes greater. For this reason, in a case where the wafer is of a large diameter or where the table diameter is so large, operating while the polishing cloth and the wafer have been pressed against each other is often employed as the operation of starting polishing.
In recent years, with the increasing diameter of wafers in a polishing process, an area of contact between the wafer and the polishing cloth has been increased, while the pressing load has also been increased to secure a desired polishing rate or a surface quality. Such an increase in the area of contact and loading causes the frictional force between the polishing cloth and the wafer to be intensified. Further, when polishing is started in the condition that the polishing cloth and the wafer have been pressed against each other, the friction, which is static friction at the instance of starting polishing, causes the maximum frictional force to be generated at that instant. Therefore, in a polishing process of the large-diameter wafer, vibrations are highly likely to occur at the instance of starting polishing, while the intensity of the vibrations thus generated being vehemently high.
The vibrations generated in the polishing process of the wafer induce cracking of wafer and cause the polishing cloth to be damaged and twisted. Because the twisted polishing cloth will damage a polished surface of the wafer, thereby inducing the generation of defects, it is necessary to replace the twisted polishing cloth and product yields get worse.
To address the problem that vibrations be generated during the polishing process of the wafer, Patent Literature 1 suggests a polishing apparatus in which a piezoelectric element is installed between bearings for a rotating shaft of a work carrier and a casing for housing the bearings. In the polishing apparatus suggested in Patent Literature 1, when vibrations are generated during the polishing process, the piezoelectric element exerts a damping force to the work carrier to attenuate the vibrations, resulting in suppressing the vibrations.
In a polishing apparatus suggested in Patent Literature 2, a fluid bearing is used in place of a ball bearing as a bearing for a table rotating shaft to suppress a possibility that a table is otherwise slightly vibrated during its rotation because balls used in the ball bearing are not truly spherical due to an avoidable production error. It is understood that in this way, the formation of microscopic ripple-shaped minute concaves/convexes on the polished surface of the wafer can be avoided, leading to an improved flatness of the polished surface.
Although a certain effect of reducing the vibrations can be expected from the polishing apparatuses suggested in Patent Literatures 1 and 2, the apparatuses become complicated in structure, and accordingly problematic in terms of facility and maintenance costs. Further, in order to adapt said polishing apparatuses to existing facilities, significant modifications should be performed for the structure of the work carrier or the table. Therefore, adapting these to the existing facilities is difficult.